1. Field of the Invention
The present invention relates to an auto-power control (APC) circuit to control the bias and the modulation currents provided to a laser diode (LD) so as to maintain the extinction ratio of the optical output from the LD, and the optical transmitter implementing the APC circuit.
2. Related Prior Art
Japanese Patent Applications published as JP-2004-087845A and JP-H11-112437A have disclosed optical transmitters with a conventional APC circuit. FIGS. 7A and 7B illustrate the conventional optical transmitter 100 and the APC circuit 116 implemented in the transmitter, which are disclosed in JP-2004-0987845A, in which the LD is coupled with the driver circuit in the AC coupling mode. The APC circuit 116 includes a monitoring photodiode (PD) 103 that converts an average optical power to a photocurrent corresponding to the average optical power and the bias generator. The bias generator, by receiving the photocurrent, sets the bias voltage provided to the LD. In this conventional APC circuit 116, external control signals and additional circuits for adjusting the temperature characteristic of the LD are necessary to adjust the bias voltage so as to compensate the dispersion of the threshold current and the slope efficiency of the LD, and the temperature dependence thereof under controlled.
FIG. 8 illustrates another conventional optical transmitter 101, which is disclosed in the JP-H11-112437A, to stabilize the average optical output power from the LD 111. The APC circuit detects the optical output by the monitoring PD 113 that generates the photocurrent IPD, and both the bias IB and the modulation currents IM provided to the LD 111 are feedback controlled by the photocurrent IPD from the PD so as to compensate the dispersion in the threshold current and the slope efficiency of the LD.
However, the APC circuits disclosed in prior documents are necessary to adjust the external reference signals to compensate the dispersion of the characteristics of individual LDs, which brings complex procedures to get a performance of the optical transmitter.